The breakdown of a gas in a cold cathode gas tube, to form a gas discharge, is primarily a statistical process depending upon the intensity of pre-ionization events and little understood differences among tubes that are ostensibly identical. If no measures are taken in the design of such tubes, turn-on delay times of many seconds are typically obtained under adverse conditions of low ambient light and low temperatures.
Various techniques have been proposed to reduce turn-on delay. See, for example, U.S. Pat. Nos. 3,792,372; 4,190,810; and 4,352,185. These are discussed at some length in U.S. Pat. No. 4,631,727 and application Ser. No. 870,794, filed 6/5/86 and allowed 1/20/88, both assigned to applicants' assignee. As indicated in these discussions, while such prior art arrangements may reduce starting time, they have a number of serious disadvantages. The first disadvantage is that they significantly increase the anode-to-ground capacitance, thereby greatly increasing the likelihood of producing parasitic relaxation oscillations. A second disadvantage has to do with cost. All these techniques tend to complicate the tube structure and add significantly to the cost.
Recently, the need for helium-neon lasers with consistently low turn-on times has led to considerable research directed at this problem. The results of this research have disclosed that micro discharges are created in such lasers outside of the preferred region of the laser, and that these micro discharges actually inhibit ignition. Accordingly, in U.S. Pat. No. 4,631,727, an electric field concentrator is incorporated into the tube design. The concentrator is located at the anode end of the tube, producing a region of high electric field intensity, thereby increasing the probability of breakdown occurring in this preferred region of the tube. This resulted in a considerable improvement in starting reliability. Typically, starting times of 150 milliseconds were realized. The maximum starting time observed was one second.
Although this proved adequate for many applications, new applications have emerged that require absolute maximum turn-on times of 0.5 second. Continuing research revealed a new variable that had previously gone undetected. The new variable is related to surface impedance variations in the glass which is a function of surface contamination effects. This discovery led to the arrangement disclosed in the above-identified application Ser. No. 890,794 wherein a coupler was added to dominate the impedance due to such contaminants.
The combination of the above-described field concentrator and the above-described coupler enabled the 0.5 second maximum starting time to be achieved with typical starting times of under 100 milliseconds. However, more recently demands upon performance have again increased. This is so in certain high-speed inspection systems where maximum turn-on times of no greater than 100 milliseconds, and average turn-on times of 50 milliseconds are needed.
It is, accordingly, the broad object of the present invention to reduce the starting time of gas lasers.